SSM2377

Rev. 0 | Page 13 of 16

THEORY OF OPERATION

OVERVIEW

The SSM2377 mono Class-D audio amplifier features a filterless

modulation scheme that greatly reduces the external component

count, conserving board space and, thus, reducing system cost.

The SSM2377 does not require an output filter but, instead, relies

on the inherent inductance of the speaker coil and the natural

filtering of the speaker and human ear to fully recover the audio

component of the square wave output.

Most Class-D amplifiers use some variation of pulse-width

modulation (PWM), but the SSM2377 uses Σ-Δ modulation to

determine the switching pattern of the output devices, resulting

in a number of important benefits.

•

Σ-Δ modulators do not produce a sharp peak with many

harmonics in the AM frequency band, as pulse-width

modulators often do.

•

Σ-Δ modulation provides the benefits of reducing the

amplitude of spectral components at high frequencies,

that is, reducing EMI emissions that might otherwise be

radiated by speakers and long cable traces.

•

Due to the inherent spread-spectrum nature of Σ-Δ modu-

lation, the need for oscillator synchronization is eliminated

for designs that incorporate multiple SSM2377 amplifiers.

The SSM2377 also integrates overcurrent and overtemperature

protection.

GAIN SELECTION

The preset gain of the SSM2377 can be set to 6 dB or 12 dB

using the GAIN pin, as shown in Table 5.

Table 5. GAIN Pin Function Description

Gain Setting (dB)

GAIN Pin Configuration

6

Tie to VDD

12

Tie to GND

POP-AND-CLICK SUPPRESSION

Voltage transients at the output of audio amplifiers can occur

when shutdown is activated or deactivated. Voltage transients

as low as 10 mV can be heard as an audible pop in the speaker.

Clicks and pops can also be classified as undesirable audible

transients generated by the amplifier system and, therefore, as

not coming from the system input signal.

The SSM2377 has a pop-and-click suppression architecture that

reduces these output transients, resulting in noiseless activation

and deactivation from the SD control pin.

EMI NOISE

The SSM2377 uses a proprietary modulation and spread-spectrum

technology to minimize EMI emissions from the device. For

applications that have difficulty passing FCC Class B emission

tests or experience antenna and RF sensitivity problems, the

ultralow EMI architecture of the SSM2377 significantly reduces

the radiated emissions at the Class-D outputs, particularly above

100 MHz. Figure 35 shows the low radiated emissions from the

SSM2377 due to its ultralow EMI architecture.

60

50

40

30

20

10

0

FREQUENCY (MHz)

+

+

+

+

FCC CLASS B LIMIT

HORIZONTAL POLARIZATION

VERTICAL POLARIZATION

Figure 35. EMI Emissions from the SSM2377

The measurements for Figure 35 were taken in an FCC-certified

EMI laboratory with a 1 kHz input signal, producing 1.0 W of

output power into an 8 Ω load from a 5.0 V supply. The SSM2377

passed FCC Class B limits with 50 cm, unshielded twisted pair

speaker cable. Note that reducing the power supply voltage greatly

reduces radiated emissions.

OUTPUT MODULATION DESCRIPTION

The SSM2377 uses three-level, Σ-Δ output modulation. Each

no input signal is present, the output differential voltage is 0 V

because there is no need to generate a pulse. In a real-world

situation, noise sources are always present.

Due to the constant presence of noise, a differential pulse is

generated, when required, in response to this stimulus. A small

amount of current flows into the inductive load when the differ-

ential pulse is generated.

Most of the time, however, the output differential voltage is 0 V,

due to the Analog Devices, Inc., three-level, Σ-Δ output modula-

tion. This feature ensures that the current flowing through the

inductive load is small.